Existing “parallel plate” or “single layer” chip capacitors are built with two parallel conductive plates, or thick film conductors, separated by a single, insulating dielectric layer that is typically made of ceramic. These single layer ceramic capacitors have a very useful form factor which renders them suitable for automated assembly into microwave frequency circuits and similar applications. Dimensions of the chip capacitors can be matched to the width of the strip lines upon which the capacitors are mounted and to which the capacitors electrically connect.
In assembly, the bottom face of the chip capacitor is typically soldered or conductive epoxy attached to a conductive surface or pad. The top face of the chip capacitor is typically ribbon or wire bonded to another connection point. Most chip capacitors have been made by metallizing opposing faces of a thin sheet of sintered ceramic. For example, gold plating is a common practice. The metallized sheet is then cut into small chip bodies by sawing or by abrasive cutting techniques.
Historically, the dielectric ceramic has been chosen for its electrical properties, with the most common material being barium titanate. In pure form, barium titanate sinters at about 1300° C. For a single capacitor, sputtering, plating or printing techniques are used to form the electrodes after the dielectric has been sintered. However, to make a multilayer capacitor where the electrodes are buried inside the chip, the electrodes are formed before the dielectric is sintered such that the ceramic must be combined with an electrode material that will not melt or oxidize at the ceramic sintering temperature. Of the four metals that will fire in air without oxidizing, namely Pt, Pd, Au and Ag, only Pt and Pd have melting points (1768° C. and 1554° C., respectively) greater than the sintering temperature. Thus, multilayer capacitors were made with pure Pd or Pt for the internal electrode plates. Eventually, however, the cost of these metals became prohibitive, such that there was a need for developing multilayer capacitors having electrodes made of alloys of these metals with Ag, which was considerably cheaper. The Ag addition, however, lowered the melting point of the electrode materials to below the sintering temperature. For example, an alloy of 70% Ag and 30% Pd has a melting point of 1150° C. So, to be able to use alloyed electrodes, the sintering temperature of the ceramic had to be lowered.
The “ultra low firing” dielectric approach, or the so-called ULF approach, is an outgrowth of traditional thick film ceramics, where materials are fired at temperatures on the order of about 880° C. to 960° C. None of the actual core materials are capable of sintering at these temperatures, but in the process they are mixed with a glass frit, which allows them to densify into a composite matrix having the desired properties of conductors, resistors or insulators.
These new ULF dielectrics sinter at less than the melting points of pure gold and silver, which are about 1064° C. and 961° C., respectively, and at less than the melting points of the AgPt and AgPd alloys. Thus, the ULF approach became useful for the production of multilayer capacitors made using pure gold, pure silver or alloyed internal electrodes.
Single layer capacitors, as opposed to multilayer capacitors, are typically mounted by wire bonding to the top metallization, and the bottom metallization is either conductive epoxy bonded or soldered. The wires used for wire bonding are typically pure gold or silver, and the best bonding results are obtained by bonding pure gold wires to pure gold electrodes and pure silver wires to pure silver electrodes. Pure metals wire bond well, whereas alloys do not. In particular, alloy additions to gold increase the hardness, thereby decreasing the reliability of the bond. Thus, the alloyed internal electrodes used in multilayer capacitors are not applicable to the top metallization of single layer capacitors. However, while very pure gold and pure silver (about 99.9%) wire bond well and can be epoxy bonded, if solder is used for the bottom connection, which is generally preferred, the molten solder will leach the gold or silver bottom metallization off the chip thereby causing a reliability problem for the capacitor. Pt and Pd, on the other hand, do not leach into solder. The Au-Sn eutectic solder is the most common solder due to its good electrical performance, but it does exhibit this tendency to leach pure gold and silver under reflow conditions.
To solve this problem, single layer capacitors have been made by sputtering gold over titanium over tungsten or by plating gold over nickel. Both sides of the chip may be wire bonded or soldered because the pure gold layer wire bonds well, and while the gold is leached into the solder, the leaching stops at the titanium or nickel layer. Therefore, multiple metal layers are needed for the metallizations if pure gold or silver top and bottom metallizations are desired, which adds to the cost of manufacturing the capacitors.
Thus, while the introduction of the ULF approach provides an advancement in the production of multilayer capacitors due to its ability to sinter at a temperature less than the melting point of desirable electrode materials, there still exists a need for reliable bonding of gold or silver metallized single layer capacitors via wire bonding and solder attachment without the above-described problem of leaching, and a method for efficiently producing single layer capacitors.